In cold welding, pressure is applied to the parts to be welded, causing plastic deformation at the surfaces of the parts in contact. The surfaces bond together forming microwelds. Under ideal conditions and in the absence of any absorbed layers of gas or liquid films, and other contaminants and oxide layers, the surfaces are close enough to each other to form interatomic bonds. This bonding mechanism can be utilized as a joining process for two dissimilar or similar materials, although, the best bonding by cold welding is typically obtained with two similar metals. When bonding two dissimilar metals that are mutually soluble, brittle intermetallic compounds may form, thus resulting in a weak joint.
Cold welding can be used in situations where it is desired to bond two parts together with a thin metallic film interposed therebetween. Thin films of a common metal can be forced together to create a joint that is strong structurally but is also so thin that it has negligible effect on the transmission of acoustic waves. For high frequency acoustic devices (e.g., ultrasonic transducers), thin metallic films are used to conduct electric current, transmit acoustic energy and provide a defect free structural bond. The metallic film provides a structural bond between the device's two primary components (e.g., a piezoelectric device and a buffer rod). A buffer rod, typically a ceramic, such as sapphire, serves as a physical support for the otherwise fragile piezoelectric device, such as, a crystal and acts as an acoustic transmission and coupling device which transmits the acoustic waves into the object or medium being tested. Indium, a relatively soft metal, exhibits good cold welding properties and is frequently used in electro-acoustic applications.
Indium cold welding requires the mating surfaces to be free of oxide formation. Therefore an inert atmosphere must be present to prevent oxidation. A high vacuum is preferred in this invention because it eliminates the risk of trapping gas pockets in the bonding operation. This implies that the coating and bonding operations must be done in a single setup inside a vacuum system without ever exposing the hardware to air until the bonding is complete.
In the prior art, stationary mounts hold the transducer crystal and the buffer rod for coating in a vacuum. When coating is complete, the two pieces are removed from the vacuum of the coating system and the pieces are bonded together in a normal atmosphere environment. This causes oxides to form on the otherwise pristine coating surfaces. As a consequence, the cold weld bond is inferior.
In some instances, an inert gas atmosphere is used to avoid oxide formation. During the bonding process, however, small pockets of gas will be trapped in the bond interface. This causes discontinuities in the bond and acoustic beam distortion.
Alternatively, the bonding can be performed inside a vacuum system without removing the parts or exposing them to the atmosphere. In this situation the prior art relied on the use of complex remote manipulators to position the components so that the mating surfaces would face each other. The bonding force is then applied usually by using a small hydraulic press. Unfortunately such hydraulic systems pose a substantial risk of contaminating the vacuum system due to fluid leakage. Further, the hydraulic fluid is supplied by an external pressure source that requires transport lines to be routed into the chamber, which restricts the allowable motion of the system within the vacuum chamber.
U.S. Pat. No. 5,148,958 to Eskandari et al. discloses a thin film vacuum cold welding system. A substrate and a transducer are translationally aligned in separate chucks and axially spaced from each other such that a bidirectional sputtering source can coat the substrate and the transducer. After the coating is complete, the sputtering source is rotated out of alignment with the substrate and the transducer. The separate chucks are then pressed together by a piston assembly to cold weld the surfaces of the substrate and the transducer together. The coating and welding processes are performed under vacuum.
U.S. Pat. Nos. 4,247,034 and 4,196,837, both to Burkart et al., disclose a method of indirectly connecting two parts, such as a quartz part and piezoelectric part, by applying relatively thin metallic layers under vacuum conditions onto the surfaces to be joined. A vacuum-chamber system having no ventilation is provided to join the surfaces together to form a permanent bond between such layers. Indium may be used as the thin metallic layer to join the surfaces together. An intermediate layer may be applied in the vacuum system onto connective surfaces of the parts to be joined prior to the application of the metallic coatings. The intermediate layer may be lead-free glass.
Visual access to high vacuum systems with multiple plating sources is very constrained and visibility is highly obscured. Without direct visual access, accurate alignment of the parts cannot be achieved. Because of this alignment uncertainty, features such as focusing lenses can only be added once the location of the transducer's acoustic beam was determined by testing. Therefore the exact location of the acoustic beam center is done only after the transducer is bonded.
The addition of the acoustic lens after bonding means that a method using a buffer rod with a preground lens could not be employed, and the possible set-up and production cost savings could not be gained. The grinding of the acoustic lens after bonding exposes the fragile piezoelectric crystal to the risk of chipping and scratching damage which frequently results in scrapped products. Consistency from unit to unit also was not easily attainable. The prior art resulted in hand-made, one-of-a-kind products that have not had widespread adoption due to low yield rates and high unit costs.